1. Field of the invention
The invention relates to a semiconductor crystal growth apparatus suited for forming monocrystalline growth layers of a semiconductor with precision as precise as a single molecular layer.
2. Description of the Prior Art
A Metal Organic Vapour Phase epitaxy process (hereinafter referred to as an MO-CVD process), a molecular beam epitaxial process (hereinafter referred to as an MBE process) and an atomic layer epitaxial process (hereinafter referred to as an ALE process) are well known in the art as vapor phase epitaxial techniques for obtaining crystalline thin film of semiconductors.
In the MO-CVD process, III and V group elements as sources, and hydrogen gas or the like as a carrier are simultaneously introduced into a reaction chamber to cause crystal growth by means of thermal decomposition. The thermal decomposition results in a poor quality of the grown crystal layer. In addition, the thickness control which is dimensionally as precise as a single monolayer is difficult.
The MBE process is well known as a crystal growth process making use of a ultrahigh vacuum, This process, however, includes a first stage of physical adsorption. Therefore, the quality of the crystal obtained is inferior to that obtained by the CVD process which makes use of a chemical reaction. Besides, for the growth of a compound semiconductor such as GaAs of III and V group elements, III and V group elements are used as sources and are disposed in a growth chamber. Therefore, it is difficult to control the amount and rate of vaporization of gases evaporated as a result of the heating of the sources. In addition, replenishment of the sources is difficult. Further, it is difficult to maintain a constant growth rate for a long period of time. Furthermore, the evacuating device is complicated in construction. Still further, precise control of the stoichiometric composition of a compound semiconductor is difficult. Consequently, the MBE process is defective in that high quality crystals cannot be obtained.
The ALE process is an improvement over the MBE process. In this process, component elements of a compound semiconductor are alternately supplied in the form of pulses so that monoatomic layers are alternately deposited on a substrate to cause growth of a thin film composed of atomic layers, as disclosed in U.S. Pat. No. 4,058,430 (1977) to T. Suntola et al. Although this process is advantageous in that the film thickness can be controlled with the precision of the atomic layer, it is actually an extension of the MBE process, and the crystal quality is not satisfactory as in the case of the MBE process. Besides, its application is limited to growth of thin films of compound semiconductors, e.g., those of II and IV group elements, such as CdTe and ZnTe, and the process is not successfully applicable to Si or GaAs, which is the most important semiconductor material presently used for the production of semiconductor devices including ultra LSI's. There are attempts for improving the ALE process so as to absorb molecules to the surface of a crystal thereby to make use of chemical reactions on the surface of the crystal. This approach, however, concerns only with the growth of polycrystals of ZnS or amorphous thin films of Ta.sub.2 O.sub.5, and has not concern with a single crystal growth technique.
With any of the prior art crystal growth processes described above, it has been difficult to obtain a crystal film of high quality and it has not been easy to control the thickness of the crystal film to a desired value.
In the meantime, in the manufacture of a semiconductor device, it is important to make evaluation during the manufacturing process as to whether or not a crystal is growing as designed for obtaining a high-quality semiconductor device. In the prior art, the evaluation has been done by taking out the semiconductor from the growth vessel and testing it using an analyzer. The operation of evaluation, therefore, has been very cumbersome, and the evaluation efficiency has not been high, resulting in incapability of attaining satisfactory quality control. Further, when a new device is to be manufactured, the evaluation has required a long time resulting in a great delay of the manufacture.
From the aspect of growth of a thin crystal film on a substrate, the surface state thereof is very important. If the surface state is unsatisfactory, the grown crystal will also have an unsatisfactory crystal property, and, in worst cases, no crystal growth is attained at all. In the case of a GaAs substrate, for instance, it has to be pretreated, prior to the crystal growth, by means of wet etching using a liquid etchant mixture consisting of H.sub.2 SO.sub.4, H.sub.2 O.sub.2 and H.sub.2 O. However, since the surface after etching is very active, an oxide layer or like deposit layer has been formed thereon, if it were exposed to atmosphere after the step of etching.